1. Field of the Invention
The present invention is related to an RFID tag information communicating device which externally reads, or writes information with respect to RFID circuit elements provided on RFID tags capable of radio-communicating information from an external unit.
2. Description of the Related Art
RFID (Radio Frequency Identification) systems for reading/writing information between compact RFID tags and reader (reading device)/writer (writing device) are known in the field. For instance, an RFID circuit element provided in a label-shaped RFID tag is equipped with an IC circuit part for storing predetermined RFID tag information, and an antenna which is connected to this IC circuit part so as to transmit/receive information. Even in such a case that an RFID tag is soiled, or an RFID tag is arranged at a position which cannot be observed, the reader/writer can access (readable/writable information) with respect to RFID tag information of the IC circuit part, and practical uses of RFID systems can be expected in various fields, for instance, commercial goods management and investigation stages.
Conventionally, in such a case that RFID tag information communicating devices access RFID tag information of IC circuit parts of RFID circuit elements in the above-explained manner, while utilizing such a characteristic that the closer an RFID circuit element is located near a device antenna, the lower the power capable of accessing the RFID circuit element is decreased, there are such RFID tag information communicating devices operated in such a manner that a signal having a certain power value is transmitted so as to recognize a tag group which responds to this transmitted signal; thereafter this tag group is brought into an idle state, another signal having a slightly larger power value is transmitted so as to recognize that another tag group responding to this transmitted signal as a tag group located far from the above-described tag group under idle state; and since the above-described operation is repeated, distributed positions of all of the RFID tags are subdivided into a plurality of regions and these RFID tags are recognized (refer to, for example, patent publication 1).
On the other hand, conventionally, in the case that predetermined information is written in the IC circuit parts of the RFID circuit elements in the above-explained manner, the below-mentioned RFID tag information communicating devices have already been proposed (refer to, for example, patent publication 2) which determines feeding speeds of RFID circuit elements along a direction of device antenna in response to printing conditions (dimensions of labels) with respect to labels adhered to the RFID circuit elements, and also, communication conditions (dimensions of tag antennas, sorts of IC circuit parts) with respect to the RFID circuit elements, while considering the following possibilities. That is, the closer an RFID circuit element is located near a device antenna, the lower the power capable of writing the information is decreased. Also, if power becomes excessively high with respect to arranging intervals of the RFID circuit elements, then the information may also be written into RFID circuit elements which are located adjacent to an RFID circuit element to be written.    Patent Publication 1: JP,A,11-344562 (paragraph numbers 0019 to 0032)    Patent Publication 2: JP,A,2003-140548 (paragraph numbers 0008 to 0022)
However, the above-explained conventional techniques own the below-mentioned problems.
In the conventional technique described in the patent publication 1, in order to recognize the distances of the respective RFID circuit elements from the device antenna, the variable control operation for controlling the transmission power from the device antenna is the major subject, and thus, such a transmission power value is determined which becomes the minimum necessary power value as long as the writing operation, or the reading operation can be carried out with respect to the respective RFID circuit elements which should be accessed. Since the access operation is carried out based upon the determined power value, this conventional technique never paid a specific attention to such a fact that leakage electric power to an external unit except for mutual spaces between the device antenna and the antenna of the relevant RFID circuit element must be suppressed as much as possible.
On the other hand, in the conventional technique described in the patent publication 2, since the feeding speed is controlled in response to the printing conditions and the communication conditions, the information is continuously written into a plurality of RFID circuit elements while the carrying operations of the RFID circuit elements are not stopped, so that it is possible to avoid lowering of the throughput. However, while the writing operations are carried out with respect to the plural RFID circuit elements in the above-explained manner, the transmission power is continuously generated from the device antenna over time durations also containing such a time period that the communications with respect to the RFID circuit elements cannot always be established. As a consequence, useless transmission power of the electromagnetic waves occurs, resulting in a lower efficiency.